1. Technical Field
The present invention relates generally to biomedical engineering and, more particularly, to systems and methods for wireless detection and communication of bioelectrical signals.
2. Description of Related Art
Electrical waveforms that carry information about the function of the heart, brain, and nervous system are useful to physicians and researchers. These electrical signals are unique in that they are naturally in the low millivolt and tens of microvolt range. They are recorded in medicine and electrophysiological research using high impedance biopotential electrodes. The use of body-invading wires to carry outside such signals from inside living things is undesirable since the wires become sites for infection as they pass through the skin and create other practical problems of breakage. Accordingly, the inventors hereof have recognized a need for systems and methods for wirelessly detecting and transmitting amplitude and waveform information from low level bioelectrical signals originating from inside the body of living things to a remote base station receiver.
There are new generations of microminiature biochemical and biophysical sensors such as for pH, pressure, and temperature, osmolarity, becoming available which would desirably be placed wholly inside the human body for remote readout. Again, with the advent of these devices the inventor hereof has found a need to communicate their signals wirelessly with the exterior of the body.
Electrical circuitry typically used to detect and transmit biosignals has required the use of high input impedance preamplifiers to boost signal levels so that they are suited to modulating transmitter and oscillator circuits of various types that might be implanted into living things. A requirement is that biotelemetry devices implanted within the human body do not use batteries but rather use passive or inductive powering techniques where incoming RF energy is rectified for powering the needs of the circuitry. The present art requires relatively complex circuitry and consequently occupies larger volumes compared to those desired for implantation in living things with minimal invasiveness. Biotelemetry devices known to the present art have been on the order of cubic centimeter volumes, and required techniques of invasive surgery to implant devices.
The inventors hereof have recognized that it be desirable to telemeter bioelectrical waveforms using devices small enough to be introduced or injected into the body through a syringe needle lumen. Unfortunately, there have been problems in reducing the size of telemetry circuits to such small sizes. There are many reasons for this but mostly they stem from the complexity of circuitry needed to transform impedances, preamplify, and modulate the bioelectrical signals onto radiofrequency signals. The present state of the art consists of using conventional semiconductor FET and transistor amplifiers to accommodate high impedance bioelectrodes and then modulating these amplified signals onto the carrier wave of small but otherwise conventional radio transmitters of various sorts. These circuits usually require methods of rectifying power from an external RF source in order to power the amplifiers and oscillator circuitry. The overall result is that wireless telemetry of low level electrical signals has required circuitry occupying significant silicon chip sizes which are undesirably large, and which require surgery to introduce into the human body.
The inventors hereof have thus devised an approach of wirelessly telemetering bioelectrical and biosensor signals that would permit very small sizes of device and would be minimally invasive and passive in design. Exemplary embodiments of the invention demonstrate the ability to wirelessly telemeter bioelectrical and biosensor signals using a passive resonant circuitry approach that provides the capabilities of impedance transformation, modulation, and radio transmission in a very simple circuitry approach suited to very small package sizes and so minimally invasive. This invention is related but not the same as techniques of the field known as RFID technology.
The communications technology broadly known as RFID is based on the property of resonant circuits whereby an electrically resonant circuit will absorb power from a nearby transmitter frequency to which it is tuned. A review of the art has been authored by K. Finkenzeller in his book RF ID Handbook, (John Wiley and Sons Inc. New York, USA) and is incorporated by reference in this document. This is a process involving the mutual coupling of inductive circuits and the concept of reflected impedance change from a remote unit to a base unit. Variations in reflected impedance is equivalent to a change in electrical loading of the base unit transmitter. This is registered by monitoring a voltage change across the transmitter resonant circuit. This voltage typically decreases as a result of increased power transfer to the remote resonant circuit and this only happens when the remote unit is in frequency resonance with the base unit. Thus, there can be an identification of the presence of a circuit tuned to the specific frequency since no reflected impedance change will occur if there is an off-resonance condition.
The process of reflected impedance occurs at frequencies that are approximately below about 30 MHz where coupling between the base transmitter and remote unit are mostly inductive. A variation on this process that is often used at higher frequencies above about 30 MHz and upward into the microwave frequency region is known as the backscatter approach. The remote resonant system is modeled as resonantly absorbing RF energy from a high frequency transmitter and re-radiating the energy causing a remote receiver to see a change in RF intensity. This change in RF backscattered intensity detected at the base unit is indicative of the local presence of a resonant circuit tuned to the transmit frequency and constitutes an identification of the circuit.
A variation of this communication approach involves the use of variable capacitance diodes incorporated into resonant circuits such that the varying capacitance of the diodes cause phase modulation as disclosed in U.S. Pat. No. 7,158,010 by Fischer et al. The use of varactor diodes also causes the emission of harmonics of the fundamental as described by Finkenzeller (op cit.). These harmonics are useful in that they can be detected and demodulated for signal information but do so at a frequency that is different from the transmitted frequency fundamental. This makes detection of the reradiated signal easier since there is no competition or interference from the fundamental frequency which is emitted from the base unit transmitter.
A more sophisticated variation of this approach is to communicate digital information by using a logic circuit that modulates the resonant frequency of the remote circuit such that it time-varies its reflected impedance or backscatter efficiency. The on-off resonance of the circuit controlled by a logic circuit can communicate to base unit digital information that uniquely identifies the circuit or information stored in logic memory. The digital logic circuitry itself can be powered by an integral battery, or if the remote circuit is close to the base unit, typically within less than a decimeter, it can rectify the incoming RF energy and use it to power logic circuits.
This prior art is directed towards identification and transmission of stored digital information by wireless methods where the battery-less and self-powered aspect of the technology permits the use of small cards and easily read formats for storage of medical records, banking and commerce, and similar purposes. Small glass encapsulated digital RFID devices for identifying animals are well known.
However, the use of resonant coupling techniques for transmission of analog information has not yet been explored. Sensors are known whereby the measured parameter, such as pressure or temperature, directly affects the tuning of the resonant circuit usually by the change of a physical parameter such as capacitor plate spacing with pressure, and hence can modulate reflected impedance or backscatter in an analog proportional way. There have also been hybrid techniques whereby amplifiers have been used with sensors and low level biopotentials to transform their typically high impedance and boost signal levels to the point where modulation of passive resonant circuits may occur. For example, Towe in 1986 demonstrated a resonant coupling method of electrocardiogram (ECG) telemetry by using high impedance preamplifiers to boost the 1 millivolt ECG to about 500 millivolts and then applying the signal through a transistor to modulate the resonance of a tuned circuit in an amplitude-proportional way. The analog signal could then be recovered at a remote base station. The preamplifiers however had the problem of requiring batteries to power the on-board amplifiers and so only used the passive resonant coupling aspect to increase battery life.
Electrical signals originating from certain types of biochemical sensors such as Nernst-type pH electrodes, are both low millivolt level and very high impedance sources, typically above about 10 megohms. For telemetry of these kinds of signals from miniature sensors using passive resonant circuit technologies, the present state of the art requires the use of buffer preamplifiers to match circuit impedances of other components and preamplifiers. These occupy space and have the requirement for on-chip dc power of the amplifiers. This is a problem in that it increases the device size and complexity and so is undesirable for minimal invasiveness into the human body.
Thus it is the purpose of this invention to provide a method for avoiding the problems of preamplification and impedance transformation of bioelectrical and biosensor signals in order to allow them to more directly and simply modulate RF backscatter produced by a simple resonant circuit system.